The development of the blue laser light source has heralded the next generation of high density optical devices, including disc memories, DVDs, and so on. FIG. 1 shows a cross sectional illustration of a prior art semiconductor laser device. (S. Nakamura, MRS BULLETIN, Vol. 23, No. 5, pp. 37-43, 1998.) On a sapphire substrate 5, a gallium nitride (GaN) buffer layer 10 is formed, followed by an n-type GaN layer 15, and a 0.1 μm thick silicon dioxide (SiO2) layer 20 which is patterned to form 4 μm wide stripe windows 25 with a periodicity of 12 μm in the GaN<1-100> direction. Thereafter, an n-type GaN layer 30, an n-type indium gallium nitride (In0.1Ga0.9N) layer 35, an n-type aluminum gallium nitride (Al0.14Ga0.86 N)/GaN MD-SLS (Modulation Doped Strained-Layer Superlattices) cladding layer 40, and an n-type GaN cladding layer 45 are formed. Next, an In0.02Ga0.98N/In0.15 Ga0.85N MQW (Multiple Quantum Well) active layer 50 is formed followed by a p-type Al0.2Ga0.8N cladding layer 55, a p-type GaN cladding layer 60, a p-type Al0.14 Ga0.86N/GaN MD-SLS cladding layer 65, and a p-type GaN cladding layer 70. A ridge stripe structure is formed in the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 55 to confine the optical field which propagates in the ridge waveguide structure in the lateral direction. Electrodes are formed on the p-type GaN cladding layer 70 and n-type GaN cladding layer 30 to provide current injection.
In the structure shown in FIG. 1, the n-type GaN cladding layer 45 and the p-type GaN 60 cladding layer are light-guiding layers. The n-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 40 and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 act as cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW layer 50. The n-type In0.1Ga0.9N layer 35 serves as a buffer layer for the thick AlGaN film growth to prevent cracking.
By using the structure shown in FIG. 1, carriers are injected into the InGaN MQW active layer 50 through the electrodes, leading to emission of light in the wavelength region of 400 nm. The optical field is confined in the active layer in the lateral direction due to the ridge waveguide structure formed in the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 because the effective refractive index under the ridge stripe region is larger than that outside the ridge stripe region. On the other hand, the optical field is confined in the active layer in the transverse direction by the n-type GaN cladding layer 45, the n-type Al0.14Ga0.86N/GaN MD-SLS cladding layers 40, the p-type GaN cladding layer 60, and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 55 because the refractive index of the of the active layer is larger than that of the n-type GaN cladding layer 45 and the p-type GaN cladding layer 60, the n-type Al0.14Ga0.86N/GaN MD-SLS layer 40, and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 60. Therefore, fundamental transverse mode operation is obtained.
However, for the structure shown in FIG. 1, it is difficult to reduce the defect density to the order of less than 108 cm−2, because the lattice constants of AlGaN, InGaN, and GaN differ sufficiently different from each other that defects are generated in the structure as a way to release the strain energy whenever the total thickness of the n-type In0.1Ga0.9N layer 35, the In0.02Ga0.98N/In0.15Ga0.85N MQW active layer 50, the n-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 40, the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65, and the p-type Al0.2Ga0.8N cladding layer 55 exceeds the critical thickness. The defects result from phase separation and act as absorption centers for the lasing light, causing decreased light emission efficiency and increased threshold current. The result is that the operating current becomes large, which in turn causes reliability to suffer.
Moreover, the ternary alloy system of InGaN is used as an active layer in the structure shown in FIG. 1. In this case, the band gap energy changes from 1.9 eV for InN to 3.5 eV for GaN. Therefore, ultraviolet light which has an energy level higher than 3.5 eV cannot be obtained by using an InGaN active layer. This presents difficulties, since ultraviolet light is attractive as a light source for the optical pick up device in, for example, higher density optical disc memory systems and other devices.
To better understand the defects which result from phase separation in conventional ternary materials systems, the mismatch of lattice constants between InN, GaN, and AlN must be understood. The lattice mismatch between InN and GaN, between InN and AlN, and between GaN and AlN, are 11.3%, 13.9%, and 2.3%, respectively. Therefore, an internal strain energy accumulates in an InGaAlN layer, even if the equivalent lattice constant is the same as that of the substrate due to the fact that equivalent bond lengths are different from each other between InN, GaN, and AlN. In order to reduce the internal strain energy, there is a compositional range which phase separates in the InGaAlN lattice mismatched material system, where In atoms, Ga atoms, and Al atoms are inhomogeneously distributed in the layer. The result of phase separation is that In atoms, Ga atoms, and Al atoms in the InGaAlN layers are not distributed uniformly according to the atomic mole fraction in each constituent layer. In turn, this means the band gap energy distribution of any layer which includes phase separation also becomes inhomogeneous. The band gap region of the phase separated portion acts disproportionately as an optical absorption center or causes optical scattering for the waveguided light. As noted above, a typical prior art solution to these problems has been to increase drive current, thus reducing the life of the semiconductor device.
As a result, there has been a long felt need for a semiconductor structure which minimizes lattice defects due to phase separation and can be used, for example, as a laser diode which emits blue or UV light at high efficiency, and for other semiconductor structures such as transistors.